Conventional converter circuits have a converter unit with a multiplicity of controllable power semiconductor switches, which are switched in a known manner in order to switch at least two switching voltage levels. Furthermore, by way of example, an LCL-filter can be connected to each phase connection of the converter unit. FIG. 1 shows one embodiment of a device for carrying out a method for operating a converter circuit according to the prior art. The converter circuit in FIG. 1 has a converter unit 1. The converter unit 1 shown in FIG. 1 is connected to an energy storage circuit 2 which is formed in the conventional manner by two series-connected capacitors. In order to operate the converter circuit, a device is provided which has a control device 15 for producing a hysteresis signal vector x, which device 15 is connected to the controllable power semiconductor switches in the converter unit 1 via a control circuit 3 for forming a control signal S from the hysteresis signal vector x. The power semiconductor switches are therefore controlled by means of a control signal S. The hysteresis signal vector x is formed by means of a hysteresis regulator 6 from a difference-phase connection current vector Δifi,i. The difference-phase connection current vector Δifi,i is in turn formed from subtraction of a phase connection current vector ifi,i from a reference phase connection current vector ifi,i,ref, with the reference phase connection current vector ifi,i,ref being formed from a reference power value Pdiff, a reference wattless-component value Qdiff and a phase flux vector ψg,αβ.
The method for operating a converter circuit described above is subject to the problem that the switching frequency of the power semiconductor switches varies to a very major extent as a result of the formation of the reference phase connection current ifi,i,ref from the reference power value Pref, from the reference wattless component value Qref and from the phase flux vector ψg,αβ. A switching frequency which is variable to such a major extent results in a significant increase in the harmonics in the phase connection currents ifg,i and in the phase connection voltages uinv,i on the converter circuit. In this context, FIG. 4 shows a corresponding time profile of a phase connection current ifg,1 which is subject to major harmonics, for one phase. By way of example, when the converter circuit is connected to an electrical interconnected power supply system, such high harmonic components are undesirable or are unacceptable. For example, when the converter circuit is connected to an electrical load, harmonics such as these can lead to damage or even destruction, and are therefore very highly undesirable.